1. Field of the Invention
The invention relates to a differential amplifier comprising first and second matched field-effect transistors (FETs) having their source electrodes connected together and to a current source, and their drain electrodes connected respectively to an input and an output of a current mirror circuit.
The invention also relates to a current sensing circuit for sensing an output current of a power semiconductor arrangement, which arrangement has a major and minor current carrying section, each section comprising at least one basic semiconductor element, said major and minor current carrying sections having a common first electrode and corresponding major and minor second electrodes.
The invention still further relates to an integrated circuit including such a current sensing circuit and a power semiconductor arrangment as described in the preceding paragraph.
2. Description of the Prior Art
Such a differential amplifier is disclosed in U.S. Pat. No. 4319181 (see FIG. 3 thereof). Two n-channel field-effect transistors (FETs) connected in the well-known "long-tail pair" configuration to form a differential amplifier. A current mirror formed by two p-channel FETs ensures that the currents in the text areas are equal with balanced inputs. The constant current source acts as a very high resistance in the "tail" of the circuit. A current sensing circuit as set forth in the second paragraph, and also a power semiconductor arrangement as set forth in the third paragraph, are disclosed in EP-A-0139998 and U.S. Pat. No. 4553084.
In a practical embodiment the known power semiconductor arrangement contains a major current-carrying section with a large number of basic semiconductor elements, such as diodes, bipolar and/or field effect transistors and thyristors. The minor current-carrying section has a significantly smaller number of such basic semiconductor elements or only one such basic semiconductor element.
It is also possible for both the major and minor current-carrying sections to contain only one basic semiconductor element, each of which however then has a different construction and/or geometry. This is the case in the cited U.S. Pat. No. 4319181 where the bipolar transistors 11 and 12 each have a lateral minor collector as well as a vertical major collector.
Power semiconductor arrangements may also make provision for several minor current-carrying sections.
In addition, control electrodes of the major and minor current-carrying sections may be connected to each other, either in the semiconductor body, or externally.
In these known power semiconductor arrangements the current level in the minor current-carrying section is assumed to be proportional to the current level in the major current-carrying section, as a result of which the current level in the major current-carrying section can be sensed by an external low power circuit because only a fraction of the output current of the power semiconductor arrangement, i.e., the output current of the minor current-carrying section, need be measured in order to be able to determine the current level in the major current-carrying section.
It has been found, however, that under certain circumstances, for example a difference between the input impedence of the current-sensing circuit and that of the load connected to the semiconductor power arrangement, the output current of the semiconductor power arrangement at the major second electrode and the measuring current at the minor second electrode are not proportional to each other.
For example, in U.S. Pat. No. 4553084, the measuring current passes through a resistance 14 to produce a voltage drop which can be measured. This voltage drop must be very much less than that across the power device, or the minor current-carrying section will be subject to different operating conditions and therefore be unrepresentative of the major current carrying section. In that citation, the voltage drop across the power device is 5 volts under a 10A load, which may be unacceptably high for many applications, for example in 12 v car systems.
Similarly, in U.S. Pat. No. 4319181, the impedance of the load is unlikely to match the impedance of the current mirror transistors 21 and 23, causing the two minor collector currents to be unrepresentative of the main load current.
In known differential amplifiers, it is usually desirable for the characteristics of the two transistors of the long-tail pair to be closely matched. Matching affords greater accuracy, and particularly improved rejection of common-mode signals.